This study compares the use and costs of services of Medicaid beneficiaries enrolled in a health maintenance organization (HMO) to those of other HMO enrollees and to Medicaid beneficiaries who receive their health care in the fee-for-service sector. Overall use and costs will be compared using 1984-85 experience. In addition, the study will examine (a) the potential selectivity of enrollment in the HMO among Medicaid beneficiaries, (b) use and costs among new Medicaid enrollees in the HMO compared to those of prior periods when they received care in the fee-for-service sector, (c) the startup effect, whereby new enrollees in HMOs use exceptionally high volumes of care, and (d) the potential selectivity of disenrollment. The study is based on a retrospective, longitudinal design covering experience from 1983 through 1985. Approximately 2,800 individuals will be sampled from each of the three populations. Data will be accumulated from existing information systems. Analyses will be based primarily on comparisons averages of indicators of use and costs.